Images captured by a camera via an imaging system, such as a lens system, will inevitably comprise several distortions. There are different kinds of distortions, and the magnitude of their occurrence will depend on a number of factors, relating to the imaging system as well as features of the imaged scene, or of a combination of the two. The present invention relates to a particular type of optical aberration, namely chromatic aberration often referred to as “purple fringing” or “blue fringing”.
The effect of purple fringing in an image is that a purple or magenta halo is formed around objects in a scene, and the effect is particularly conspicuous around edges where a dark area meets a bright area in the image. Chromatic aberration may be attributed to the fact that light of different wavelengths will refract differently when passing a refractive medium, more particularly a lens of an imaging system. For a lens, there are two types of chromatic aberration; axial, or longitudinal, chromatic aberration and lateral, or transverse, chromatic aberration. The axial chromatic aberration corresponds to that light of different wavelengths travelling parallel to an optical axis as it falls in on the lens will have a focal point at different positions along the optical axis. As a rule, the shorter wavelengths will have their focal points closer to the lens than light of longer wavelengths. The lateral chromatic aberration corresponds to that light falling in obliquely to the lens will have their focus in the same plane, but in different positions in that plane.
The effect of longitudinal chromatic aberration is colored areas in the image, since not all three colors (red, green, blue) may be imaged in focus at the same time, giving edges in the image a smeared appearance. The effect of lateral chromatic aberration may be more or less sharp fringes, particularly visible around edges and objects of high contrast in the image.
Different ways of eliminating or minimizing the effects of chromatic aberration are available in prior art, basically ranging from composing lenses of the imaging system in an appropriate way, thus forming an achromat, to post-processing of the images to remove the effects, of which the latter is disclosed in US2012/201454.
It is also known that various types of processing of images, having the purposes of improving the image in other ways, may enhance the adverse effects of chromatic aberration making blue fringing more prominent in the image. An example is the use of imaging with wide dynamic range (WDR) or high dynamic range (HDR), where a single image is the result of a longer and a shorter exposure image fused or merged together so as to increase the dynamic range of the resulting image.
The present invention aims at circumventing the effects of chromatic aberration in a straightforward manner, and which may be applied to a live stream of images, such as the images acquired with a digital video camera. As an added benefit, the invention may also reduce the effect of other distortions resulting in a chromatic error.